In a memory system that uses NAND type flash memory for a storage medium, data is stored with an ECC (Error Correction Code/Error Checking and Correction) of a predetermined length to be used for error correction added to the data of a predetermined length. However, consideration is not conventionally taken on a case in which degradation in a bit error rate (hereinafter referred to as BER) of a block, which is a unit of erase, is found.